Improved microstructure and compressive strength of pastes and mortars containing MgO-SiO2 cement produced by combined calcination of MgCO3 and kaolin

Abstract

MgO-SiO₂ cement was produced by combined calcination of MgCO₃ and kaolin as raw materials, at a MgCO₃/kaolin mass ratio of 3.0 and 800 °C for 45 min. Separate calcination of MgCO₃ and kaolin was also conducted under the same conditions. Raw materials were characterized based on their chemical composition, bulk density, particle size distribution, XRD, FTIR, and BET specific surface area, and the MgO-SiO₂ cement produced was analyzed using XRD, FTIR, SEM/EDS, BET specific surface area, and particle size distribution. Pastes were produced to characterize microstructure and the reaction process (XRD, FTIR, TGA, isothermal calorimetry, chemical shrinkage, and SEM/EDS), and mortars to assess compressive strength. The results of BET and SEM/EDS analyses of the cements confirmed that combined calcination increased particle adhesion. Microstructural analyses of pastes after 28 days of curing at 25 °C showed the formation of a dense matrix of M-A-S-H reaction product, with aluminum incorporated into the M-A-S-H structure; brucite and hydrotalcite were also generated after the reaction process. Paste reaction results demonstrated that combined calcination produced cement with a higher degree of reaction after 72 h at 25 °C. The highest compressive strength (42.0 MPa) in mortars obtained by combined calcination was recorded after 28 days of curing at 25 °C, 223 % higher than that of separately calcined cement (13.0 MPa). As such, this study successfully applied combined calcination to produce a more strength MgO-SiO₂ cement.